Inserting green screen footage into 3D space
Probably the best way, is to key out the green, and then import the image sequence into the 3D view.

Start by taking the image sequence into Blender, or your compositor of choice, and key out the green to your taste. Try to keep in mind the environment it's going to end up in and make sure that the key will reflect that. Then render out the frames as a PNG sequence, or any other image file format that Blender accepts that uses transparency.

In Blender, use the import image as planes addon (included by default) to import the image sequence into the 3D view. Make sure that alpha is enabled, and shadeless is checked in the settings before importing.

(You want to have shadeless checked, because if it interacts with light, it will be treated as a flat object, so any shadows that are there will be completely flat.)

Then place the plane where you want it to be, and use the color tools to make it fit into the environment properly. Or use some render layers magic to render it out separate from everything else to allow you to composite it back in properly either useing Blender or an external compositor.